The present invention concerns spatial filtering of video images and, in particular, an adaptive filtering system which switches between field-based and frame-based filters on a block-by-block basis responsive to motion in the corresponding blocks of the video image.
It is often desirable to filter interlaced video signals for many different applications. For example, if the image is to be reduced in size, it is desirable to low-pass filter the image, both horizontally and vertically, to prevent aliasing distortion in the subsampled image. For image data compression applications it may also be desirable to band-limit the video signal prior to compression to reduce the visibility of artifacts arising from the image compression process.
A purely horizontal filter is fairly simple to implement since it involves temporally filtering successive picture elements (pixels) on each line of the interlaced video signal. Vertical filtering of an image signal, however, is more complex since there are problems implementing a vertical filter, either on a purely frame basis or a field basis.
To understand these problems, it is helpful to describe the structure and timing of an interlaced video signal. FIG. 1a illustrates an image frame of, for example, an NTSC video signal. The exemplary frame exhibits the highest vertical resolution that may be obtained in such a signal. This is achieved by having alternate lines, A, of the image at a different signal level than the intervening lines, B. Since there are 480 lines in the active portion of the NTSC video signal, the maximum vertical resolution that can be achieved is 240 cycles per picture height (CPH) as dictated by the Nyquist criteria.
A typical NTSC video signal, however, is neither transmitted, received nor displayed as shown in FIG. 1a. Instead, the signal is transmitted as two fields shown in FIGS. 1b and 1c respectively. As shown in FIG. 1b, all of the lines of the first field have the level A while, as shown in FIG. 1c, all of the lines of the second field have the level B. Due to persistence of vision in the human eye, the two fields, having a display rate of 60 fields per second, are integrated into the frame image shown in FIG. 1a, having a repetition rate of thirty frames per second.
If there is no motion in the image represented by the video signal, that is to say, if the pixels in the image are the same from frame to frame, it is preferable to filter the image as a frame instead of as two fields. This is illustrated by the frame and fields shown in FIGS. 1a, 1b and 1c. If the image frame shown in FIG. 1a were vertically filtered by a filter having a cut-off resolution of, for example, 180 CPH, the result would be a frame image having lower resolution than the original frame image. If, however, the fields shown in FIGS. 1b and 1c were passed through the same filter, there would be no difference between the image produced by the input signal and that produced by the output signal. This can be seen because there is no high-resolution component in either of the two fields while there is a high-resolution component in the single frame which results from combining the two fields.
On a more theoretical level, FIG. 2a shows the passband/stopband spectrum of a frame vertical filter for the case where the cut-off resolution is 180 CPH. As can be seen in FIG. 2a, the shaded region, representing vertical frequencies passed by the filter, is limited between 0 and 180 CPH. Image components having vertical resolution between 180 CPH and 240 CPH are not passed by this vertical filter. FIG. 2b shows this same filter applied on a field basis. As can be seen, the integrated frame image generated from this filtered field signal has a stopband between 90 and 150 CPH and a passband elsewhere. This is obviously not a low-pass filtered characteristic.
On moving images, field filtering may still be more desirable than frame filtering because it substantially reduces temporal artifacts which may be caused by frame filtering.
For example, FIG. 3 illustrates how the image of a box moving to the right would be represented by an interlaced video signal. The outline of the box in the first field is illustrated by the solid line and, in the second field, by the broken line. When fields one and two are combined to form a frame, regions of serrations (i.e. spurious alternate lines) appear along the vertical edges of the box. These serrations occupy a horizontal dimension equal to the motion of the box in one field period. If these image fields were to be combined into a single frame, frame filtered and then displayed, these artifacts would be displayed, albeit at a reduced resolution in each field of the frame.
If, however, the image were processed as two successive fields, there would be no temporal artifacts, each field image would have the object in its proper position in the time sequence. Obviously, such temporal artifacts are to be avoided; therefore, in moving areas of an image, field filtering may be preferred to frame filtering.